In the Laboratory
Designing and Conducting a Purification Scheme as an Organic Chemistry Laboratory Practical Kate J. Graham,* Brian J. Johnson, T. Nicholas Jones, Edward J. McIntee, and Chris P. Schaller Department of Chemistry, College of Saint Benedict/Saint John’s University, St. Joseph, MN 56374; *
[email protected] Students must develop practical skills in the organic chemistry laboratory, but building these skills through “cookbook” procedures does little to prepare students for more realistic situations in a professional laboratory (1). Students who have been trained to follow directions cannot decide how to adapt a known technique to new situations and may not even know which technique they should use to purify a mixture (2–4). Although good lab skills are crucial, the ability to devise a plan of action when faced with a familiar or slightly unfamiliar scenario is equally important (5). By introducing a laboratory practical in our curriculum, we hoped to accomplish a number of goals. We sought to motivate students to practice better techniques, which is a primary reason lab practicals have always been used (2, 6–9). In addition, we wanted to examine the students’ rationale for choosing a particular manipulation in a given situation, rather than solely judging how well they performed an assigned task (3). We tried to design an exercise in which, upon entering the laboratory, each student was on her or his own both in deciding how to approach a given problem and in carrying out that approach (10). We also hoped students would be more able to cope with synthesis projects, in which the purification steps often pose big hurdles, so we wished to present situations that could occur in carrying out a reaction (11). Finally, we wished to assess whether students could use data analysis to determine whether they had been successful (7, 12). Overall, the goal of this lab practical was to test how well students could integrate all the tasks expected of them in a laboratory; for example, designing an experiment, carrying out an experiment, and analyzing data (3). The focus of the practical was on purification techniques, without which students can make little headway in more advanced synthetic experiments. Outline of the Lab Practical The laboratory practical was scheduled for late in the first semester of organic chemistry. At that point, students had completed all of their standard laboratory manipulations: a recrystallization, a liquid–liquid extraction, an acid–base extraction, a simple distillation, a steam distillation, and column chromatography. Students had also carried out each of the following analytical techniques one or more times: melting point analysis, IR spectroscopy, NMR spectroscopy, polarimetry, thin-layer chromatography, and gas chromatography–mass spectrometry. To help students prepare for the lab practical, our in-house lab manual provided a list of approximately thirty scenarios where a sample needed further purification. Each of these descriptions corresponded to a possible assignment students could
1644
receive for the lab practical. For example, one scenario was During an oxidation of benzyl alcohol to benzoic acid, a student did not have adequate time during the lab period to let the reaction mixture stir long enough. Consequently, the benzoic acid he obtained was still contaminated with benzyl alcohol. Purify the benzoic acid and demonstrate its final purity.
Before coming to lab, students were asked to prepare complete summaries of each purification technique they had studied, including the circumstances in which each one would be useful. They were also asked to summarize what kind of information could be obtained from each method of characterization they had used. On their own, they were encouraged to develop a plan for each of the scenarios outlined in the lab manual. Although it was clear that students would discuss these scenarios among themselves beforehand, once in lab they were not allowed to talk to each other. They could not ask questions of the instructor or the teaching assistant (TA), although they were allowed to request that the TA obtain stockroom supplies. As a result, students were left to fail or succeed on their own. Students selected their own exercise blindly by removing from a bag a vial with an attached slip of paper describing the supposed origins of the material contained in the vial. Based on the information given, the students had to decide how to purify the contents of the vial. The experiment was done in microscale to minimize hazards. Typically, this was a 100–200 mg sample containing about 75% of the desired compound and 25% of an impurity. Students were allowed to use their laboratory text or handbooks containing physical data on organic compounds, but not Web-based sources. IR and NMR spectra were expected of the purified compound as well as any additional data that would show whether the purification was successful. The exercise had a time limit of four hours. After the laboratory period, students used their data to assess whether their purification schemes had worked properly and submitted a written report. The report is based on the Journal of Organic Chemistry style and consisted of a results and discussion section, an experimental section, both raw and tabulated data, but no abstract or introduction. The deadline for the report was one week after completion of the experiment. Instructors still could not answer questions about the experiment, in order to avoid interfering with the experience in other lab sections. The grade for the report was dependent upon the presence of sufficient and appropriate data, the choice of purification technique, appropriate data analysis (including structure determination and an argument about whether the purification had worked based on the evidence) and success of purification as judged by the instructor (see the online material). The purpose of this grading
Journal of Chemical Education • Vol. 85 No. 12 December 2008 • www.JCE.DivCHED.org • © Division of Chemical Education
In the Laboratory
scheme was to balance different aspects of the laboratory experience; a student who made a poor choice of what technique to use would be penalized, but only to a point, provided she or he gathered appropriate data and could recognize what that data meant. On the other hand, a student who executed the experiment perfectly but could not present a convincing case would not get full points, either. Outcome and Student Response A survey solicited student opinions on different aspects of the exercise. Notably, students deemed “figuring out what purification approach to use” the most difficult part of the lab practical (46.6% or 54/116 responses ). “Effectively carrying out the procedure in lab” was considered the next most challenging at 40.5% (47/116 responses) followed by “interpreting spectral data” (12.9% or 15/116). These data suggest that the design of the lab practical is targeting a genuine need as perceived by the students. Day-to-day communication between students did not seem to be a problem. However, students often worked in groups ahead of time and prepared spreadsheets with exact, step-by-step procedures for each scenario. To ensure some independence, students’ notebooks were checked in lab to make sure they did not bring detailed procedures for specific scenarios with them. However, they were allowed to use their notebooks to follow procedures they had performed previously. Implementation
the solvents used include suspected carcinogens or mutagens (dichloromethane and methyl tertiary butyl ether) and should not be employed outside a fume hood. MSDS information should always be consulted before implementing a new experiment. Conclusion An open-ended laboratory practical has been successful in challenging students to focus on the development of useful techniques in the organic lab and to evaluate when different techniques would be appropriate. In contrast to most lab practicals, the overall grade on the practical experiment takes spectral analysis of real data as well as writing skills into account, but a significant portion of the grade lies in successful execution of a purification technique. Most importantly, being able to decide upon the best approach to purification is a crucial component of the exercise, forcing students to be intellectually engaged rather than simply follow directions. Literature Cited
1. 2. 3. 4. 5.
6. 7.
Monts, D. L.; Pickering, M. J. Chem. Educ. 1982, 59, 1032. King, M. M. J. Chem. Educ. 1974, 51, 125–126. Casanova, J.; Tunstad, L. Chem. Educator 1996, 1, 1–4. Leonard, J. E. J. Chem. Educ. 1981, 58, 1022. Hilosky, A.; Sutman, F.; Schmuckler, J. J. Chem. Educ. 1998, 75, 100. Frey, J. E. J. Chem. Educ. 1994, 71, 51–53. Silberman, R.; Day, S.; Jeffers, P.; Klanderman, K.; Phillips, M. G.; Zipp, A. J. Chem. Educ. 1987, 64, 622–623. Harris W. E.; Kratochvil, B. J. Chem. Educ. 1971, 48, 543–544. Neeland, E. G. J. Chem. Educ. 2007, 84, 1453. Rozeiu, A. M. J. Chem. Educ. 1969, 46, 120. Graham, K. J.; Schaller, C. P.; Johnson, B. J.; Klassen, J. B. Chem. Educator 2002, 7, 376–378. Robyt, J. F.; White, B. J. J. Chem. Educ. 1990, 67, 600–601.
There is no need to limit the possible scenarios for this lab practical to those that we describe in the online material. A scenario could involve any mixture that can easily and safely be separated by a student along with a fictional story about the origin of the mixture. In addition, students were supplied with some common solvents such as methanol, tert-butyl methyl ether, hexanes, and ethyl acetate, as well as some dilute acids and bases and drying agents.
8. 9. 10. 11.
Hazards
Abstract and keywords
Because of the use of microscale, the potential hazards of this experiment are not significant. However, it should be noted that many of these compounds (acetylsalicylic acid, resorcinol, ibuprofen, methylsulfonic acid, boric acid, and benzoic acid) are corrosives and contact with skin should be avoided. In addition,
Full text (PDF) with links to cited JCE articles
12.
Supporting JCE Online Material
http://www.jce.divched.org/Journal/Issues/2008/Dec/abs1644.html
Supplement
Representative examples of scenarios for the lab practical
Grading rubric
© Division of Chemical Education • www.JCE.DivCHED.org • Vol. 85 No. 12 December 2008 • Journal of Chemical Education
1645
